Since the fuses of the present invention were developed for use with ozonators, some background regarding ozonators is of particular significance. Ozone is produced by passing air, oxygen, or a mixture of gases containing oxygen through an energy field called a "corona discharge". The corona discharge is generated by creating a voltage potential between a series of mediums of known dielectric constant, one of which is the gap through which the oxygen containing gas is passed. The voltages encountered in an ozonation are generally from 6 KV to 22 KV and the frequencies from 50 Hz to 10,000 Hz.
Ozonators are often constructed of a group of cells. Each cell has one or more electrodes bonded or otherwise secured to a dielectric, an air gap and a ground plane. The ground plane frequently serves as one surface of a heat exchanger since most of the energy passed through the corona discharge is converted to heat and must be dissipated through air to air, air to water, or air to other cooling medium heat exchangers.
Failure of a dielectric results in the rapid discharge of energy through the dielectric at the point of failure. Ozonator power supplies are generally provided with thermal and magnetic circuit breakers or other devices that trip when a dielectric failure occurs. Since ozonator dielectrics are connected in a parallel arrangement and each draws a fixed amount of current under normal operating conditions the greater the number of dielectrics, the higher the trip point current on the ozonator breaker, and the greater the total capacitance of the ozonator. Thus, as the size of the ozonator increases, the energy available for discharge through a failed dielectric increases, while the ability to detect a failed dielectric and quickly shutdown the system becomes more difficult.
Failure to either shut down the individual failed dielectric or the ozonator usually results in puncture of the ground plane at the point of dielectric failure. Since the ground plane is usually one surface of the heat exchanger used for cooling the unit, puncture of the ground plane causes the cooling medium to mix with the ozonized air or oxygen.
In generators using water for the cooling medium, water will enter the ozonation cells if the pressure in the water jacket is higher than the air pressure in the ozonation cell. In an oil cooled system, oil can enter the ozonation cell and, depending on the flamability of the oil, the feed gas, and the process application, fire and/or process contamination can result. In air cooled systems the pressure inside the ozonator cell is greater than the pressure in the heat exchanger and ozonized air is blown out the cooling system discharge. In all cases dielectric failure requires that the unit be shut down, the failed dielectric removed from service, and, in the event of ground plane puncture, the unit be taken out of service and repaired.
In an attempt to prevent ground plane puncture and need to disassemble an ozonator to remove a failed dielectric from service, the applicant's assignee Welsbach developed a fuse in 1965. This fuse was manufactured from 1965 to 1980 and used in air and oxygen service and consisted of a meltable fuse link connected between two terminals which were supported relative to one another by a standoff insulator of some sort, such as a ceramic rod. However, there were several majors problems with the original Welsbach fuse that the present invention corrects.
The fuse element was a standard 3/4 ampere buss fuse wire and was subject to deterioration in the ozone atmosphere and subsequent failure.
At low voltages, 6 to 8 KV, the fuse did not always blow when a dielectric failure occured and consequently ground plane punctures occured.
When a dielectric failure occurs in large ozonators at any voltage the energy dump through the fuse causes rapid oxidation to the point of vaporization of the fuse and plating of the fuse metals onto the insulator supporting the fuse terminals. This plating in many cases allowed sufficient current to flow through the fuse to puncture the ground plane at the point of dielectric failure without interrupting the oxonator circuit breaker until after the damage was done.